The invention relates to a spherical-aberration detection system for measuring spherical aberration of an optical beam, to an optical device for scanning record carriers using the detection system and to a device for measuring the thickness of a transparent layer.
The measurement of spherical aberration has recently become relevant in the field of optical recording. The information density on optical record carriers may be increased by increasing the numerical aperture (NA) of the radiation beam used for reading and writing information on the record carrier. Record carriers are often scanned through a transparent layer protecting the information layer of the record carrier. A small variation of the thickness of the transparent layer causes a substantial change in the spherical aberration incurred by a high-numerical aperture radiation beam traversing the transparent layer. This spherical aberration may be reduced by using a dual lens objective system. Such a system has a first lens and a second lens, the second lens being a plano-convex lens arranged between the first and lens and the record carrier, and a small spacing between the plano surface and the record carrier. In some applications the plano-convex lens is referred to as a solid immersion lens.
The article "High density optical disk system using a new two-element lens and a thin substrate disk"by F. Maeda et al, published in the proceedings of ISOM96 p. 342-344 discloses an optical recording system having such a dual-lens objective system. The spherical aberration due to variations in the thickness of the transparent layer are compensated by changing the axial position of the plano-convex lens of the objective system. The system determines the spherical aberration in the beam reflected from the record carrier and uses this value to position the plano-convex lens. The article proposes two methods to determine the amount of spherical aberration in the reflected beam. In the first method, the envelope magnitude of the information signal read from the record carrier is measured, which will have a maximum value when the spherical aberration is at a minimum value. In the second method the shape of the focus error signal as a function of the focus error is analyzed, and the position of the plano-convex lens is optimized to obtain the desired shape.
The above citations are hereby incorporated herein in whole by reference.